Shorewall and UPnP

TomEastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
“GNU Free Documentation
License”.

UPnP

Shorewall includes support for UPnP (Universal Plug and Play) using
linux-igd (http://linux-igd.sourceforge.net).
UPnP is required by a number of popular applications including MSN
IM.

Warning

From a security architecture viewpoint, UPnP is a disaster. It
assumes that:

All local systems and their users are completely
trustworthy.

No local system is infected with any worm or trojan.

If either of these assumptions are not true then UPnP can be used
to totally defeat your firewall and to allow incoming connections to
arbitrary local systems on any port whatsoever. In short: USE UPnP
AT YOUR OWN RISK.

Important

Shorewall and linux-igd implement a UPnP Internet
Gateway Device. It will not allow clients on one LAN subnet
to access a UPnP Media Server on another subnet.

linux-igd Configuration

Shorewall Configuration

In /etc/shorewall/interfaces, you need the
'upnp' option on your external interface.

Example:

#ZONE INTERFACE OPTIONS
net eth1 dhcp,routefilter,tcpflags,upnp

If your loc->fw policy is not ACCEPT then you need this
rule:

#ACTION SOURCE DEST
allowinUPnP loc $FW

You MUST have this rule:

#ACTION SOURCE DEST
forwardUPnP net loc

You must also ensure that you have a route to 224.0.0.0/4 on your
internal (local) interface as described in the linux-igd
documentation.

Note

The init script included with the Debian linux-idg package adds
this route during start and deletes it during
stop.

Caution

Shorewall versions prior to 4.4.10 do not retain the dynamic rules
added by linux-idg over a shorewall restart.

If your firewall->loc policy is not ACCEPT, then you also need to
allow UDP traffic from the fireawll to the local zone.

ACCEPT $FW loc udp - <dynamic port range>

The dynamic port range is obtained by cat
/proc/sys/net/ip_local_port_range.

Shorewall on a UPnP Client

It is sometimes desirable to run UPnP-enabled client programs like
Transmission
(BitTorrent client) on a Shorewall-protected system. Shorewall provides
support for UPnP client access in the form of the upnpclient option in shorewall-interfaces
(5).

The upnpclient option causes
Shorewall to detect the default gateway through the interface and to
accept UDP packets from that gateway. Note that, like all aspects of UPnP,
this is a security hole so use this option at your own risk.

Note that when multiple clients behind the firewall use UPnP, they
must configure their applications to use unique ports.